1. Field of the Invention
The present invention relates to noise attenuation panels and to a method of manufacturing a noise attenuation panel.
2. Background
Noise attenuation panels are widely used for attenuating noise produced by aircraft engines and are located at optimised positions in the flow ducts of aircraft engine nacelle structures. Such flow ducts primarily comprise the inlet duct, the fan duct and the nozzle assembly.
A typical noise attenuation panel comprises a sound reflecting solid backing plate or sheet, a perforate metal facing plate or sheet and a honeycomb or cellular core which is bonded between the backing and facing sheets and which partitions the air into a multiplicity of separate cells.
When the noise attenuation panel is mounted in a flow duct of an aircraft engine nacelle structure with the facing sheet exposed to sound waves generated in the duct, the sound waves become subjected to three mechanisms which result in a reduction of the sound energy by conversion thereof to heat energy, namely:
(i) friction in the facing sheet,
(ii) pressure loss when duct pressure sound waves expand into the cells of the honeycomb or cellular core, and
(iii) xe2x80x98reactivexe2x80x99 cancellation of the direct incident sound wave by the wave that is reflected from the solid backing sheet, the honeycomb cell depth being xe2x80x98tunedxe2x80x99 to the required frequency.
Noise attenuation panels are obviously important acoustically but, because of the hostile environment in which they operate, there is also an evident need for structural rigidity. As they form part of an aircraft engine nacelle structure it is important that the complete component is provided with adequate strength to withstand the inflight conditions to which an exposed area of a nacelle structure is susceptible. Moreover, noise attenuation panels are often so configured as to add strength to the nacelle structure into which they are to be installed.
The perforate facing sheets of noise attenuation panels heretofore proposed have commonly been perforated by punching or mechanical drilling. Current noise attenuation panel constructions use perforate facing sheets with holes typically of diameter between 0.020xe2x80x3 (0.508 mm) and 0.060xe2x80x3 (1.524 mm)positioned in an equi-spaced triangular array such as to provide open areas within the limits of 3 and 20%.
Early manufacturing procedures have included punching and drilling but they do not realistically allow for the provision of very small hole diameters and very small spacing. In metal sheets, punching for example dictates a minimum hole diameter of 0.020xe2x80x3 (0.508 mm). Mechanical drilling can produce holes with diameters as low as 0.010xe2x80x3 (0.254 mm), but this is highly impractical with the lowest practical hole diameter being 0.020xe2x80x3 (0.508 mm).
In prior patent specification GB 2314526, a method of manufacturing a noise attenuation panel has been proposed in which a blank facing sheet is subjected to electron beam drilling to produce a multiplicity of drilled holes with bore diameters not greater than 0.020xe2x80x3 (0.508 mm). The electron beam drilling it is stated may furthermore advantageously produce the multiplicity of drilled holes with bore diameters in the range of 0.002xe2x80x3 (0.0508 mm) to 0.020xe2x80x3 (0.508 mm).
It has furthermore been proposed in prior patent specification U.S. Pat. No. 4,850,093 to provide a perforated titanium facing sheet by laser drilling holes or perforations through the sheet. The holes are uniformly distributed over the sheet and constituted 3 to 6% of the total area of the sheet. The porosity of the facing sheet may, it is stated, be designed to meet specific flow resistances by either changing the hole size or the spacing between holes or by simultaneously changing both. A facing sheet considered to be suitable comprised holes with a hole diameter of 0.002 to 0.003xe2x80x3, a hole spacing of 0.008 to 0.016xe2x80x3, 11,000 to 16,000 holes per square inch and 3 to 6% open area.
In prior patent specification GB 2038410A it has been proposed to provide a noise attenuation panel for a fluid flow duct of a gas turbine aeroengine which is aimed at attenuating as many frequencies as possible by employing beneath the perforated facing sheet a Helmholtz-type resonator for frequencies at the lower end of the frequency range and tube-type resonators for higher frequencies. Attention is directed to varying the Helmholtz resonator characteristics to provide for a wide band absorption. The facing sheet has a regular array of uniformly-sized holes although it is proposed to increase the hole density by reducing the spacing between the holes at one location of the facing sheet for acoustic coupling purposes.
In prior patent specification U.S. Pat. No. 4,288,679 a laser microdrilling method is proposed which uses a power laser beam in which the surface finish and dimensional accuracy of the hole being formed is improved by heating the workpiece. The laser beam can, it is stated, be rotated about the axis of the beam to form holes having a surface finish with a dimensional accuracy better than those obtainable with conventional laser microdrilling technology.
It is an object of the present invention to provide a noise attenuation panel and a method of manufacturing the panel in which the holes in the facing sheet can, by virtue of their hole geometry and distribution, provide for noise attenuation over a wide range of frequencies to which the panel is subject when employed as a noise attenuation panel for gaseous flow ducts in gas turbine aeroengines.
The present invention according to its different aspects includes a noise attenuation panel or the manufacture of a noise attenuation panel which comprises: a cellular component part which has a front face and a rear face and a cell defining wall structure which defines a multiplicity of cells between the front face and the rear face; and a facing component part which has a front face and a rear face, extends across the ends of the cells of the cellular component part at the front face thereof with the rear face of the facing component part adjacent the front face of the cellular component part, and is provided with a multiplicity of holes which extend through the facing component part from the front face to the rear face to provide gaseous fluid communication between the cells of the cellular component part and the front face of the facing component part for the attenuation of noise generated by gaseous fluid flow over the surface of the front face of the facing component part.
According to a first aspect of the present invention, there is provided a method of manufacturing a noise attenuation panel as hereinbefore set forth, the method comprising producing, in a hole producing step, the multiplicity of holes through the facing component part in the form of an array of holes having a hole size which so varies over the facing component part as to provide optimum attenuating performance of the panel over a predetermined range of gaseous flow conditions at the front face of the facing component part.
According to a second aspect of the present invention, there is provided a method of manufacturing a noise attenuation panel as hereinbefore set forth, the method comprising producing, in a hole producing step, the multiplicity of holes through the facing component part in the form of an array of holes having a non-circular hole cross-section which is so chosen as to provide optimum combined structural strength and attenuating performance of the panel.
According to a third aspect of the present invention, there is provided a method of manufacturing a noise attenuation panel as hereinbefore set forth, wherein: the facing component part is in the form of a fibre reinforced composite comprising a matrix component and a fibre reinforcing component embodied within the matrix component; the fibre reinforcing component comprises arrays of fibres in which the fibres of each array extend in a predetermined direction in the matrix and in which the predetermined direction of the fibres of each array is different from that of the fibres of each of the other arrays; and the holes in the facing component part are so produced as to have a polygonal cross-section with sides arranged parallel to the predetermined directions of the fibres of the arrays.
According to a fourth aspect of the present invention, there is provided a method of manufacturing a noise attenuation panel as hereinbefore set forth, in which the cell defining wall structure comprises walls which define the multiplicity of cells and which terminate in end portions at the front face of the cellular component part, and drilling of the holes is so carried out that no holes are drilled at locations of the facing component part which in the assembled panel are contiguous with the end portions of the walls of the cell defining wall structure.
According to a fifth aspect of the present invention there is provided a method of manufacturing a noise attenuation panel as hereinbefore set forth, the method comprising producing, in a hole producing step, the multiplicity of holes through the facing component part in the form of an array of holes which pass through the component part from the rear face of the facing component part to the front face of the facing component part in a predetermined hole direction inclined to the normal to the front face.
In an embodiment of the invention the multiplicity of holes through the facing component part is in the form of an array of holes which pass through the component part from the rear face of the facing component part to the front face of the facing component part in a predetermined hole direction inclined to the normal to the front face. The inclination of the holes is so chosen as to provide flow paths to the cell defining structure which optimise attenuating performance of the panel.
Where the facing component part is a multi-ply structure comprising a plurality of superposed ply elements the holes may be so inclined as to offset structural weakness of the multi-ply structure in the region of the holes.
Where the panel is so located as to be subjected to gaseous fluid flow over the surface of the front face of the facing component part in a predetermined fluid flow direction the predetermined hole direction has a component along the front face of the facing component part which is in the same direction as the predetermined fluid flow direction. Furthermore, the holes may be so inclined as to reduce the tendency of the holes to become clogged by debris carried in the gaseous fluid flow over the front face of the facing component part.
According to a sixth aspect of the present invention there is provided a noise attenuation panel as hereinbefore set forth which is produced by the method according to its first, second, third and/or fourth aspect.
According to a seventh aspect of the present invention there is provided a noise attenuation panel as hereinbefore set forth which is characterised in that the multiplicity of holes through the facing component part form an array of holes having a hole size which so varies over the facing component part as to provide optimum attenuating performance of the panel over a predetermined range of gaseous flow conditions at the front face of the facing component part.
According to an eighth aspect of the present invention, there is provided a noise attenuation panel as hereinbefore set forth which is characterised in that the multiplicity of holes through the facing component part form an array of holes having a non-circular hole cross-section which is so chosen as to provide optimum combined structural strength and attenuating performance of the panel.
In an embodiment of the invention, the holes of the array have a non-circular hole cross-section which varies over the facing component part.
Furthermore, in an embodiment of the invention, the holes of the array are spaced apart with a hole spacing which varies over the facing component part.
In embodiments of the invention hereinafter to be described, the holes in the facing component part are produced by laser drilling.
In an embodiment of the invention, the method includes bringing the two component parts together in an assembly step in the formation of the noise attenuation panel, and carrying out the hole producing step prior to the assembly step. The laser drilling is then carried out using a high intensity laser which may take the form of a CO2 or YAG laser.
In an embodiment of the invention, the method includes bringing the two component parts together in an assembly step in the formation of the noise attenuation panel, and carrying out the hole producing step after the assembly step. The laser drilling is then carried out using a low intensity laser which may take the form of a UV Excimer laser.
In an embodiment of the invention, the panel further comprises a backing component part extending across the rear face of the cellular component, and the assembly step comprises bringing the three component parts together in the formation of a noise attenuation panel.
In an embodiment of the invention according to its fourth aspect, the method includes bringing the two component parts together in an assembly step in the formation of the noise attenuation panel, carrying out the hole producing step after the assembly step, and carrying out laser drilling under the control of an ultrasonic probe identifying wall end portion locations.
In an embodiment of the invention, the holes of the array have bore configurations which minimise blockage by debris deposited by the gaseous flow therethrough.
According to a ninth aspect, there is provided a noise attenuation panel as hereinbefore set forth, wherein: the facing component part is in the form of a fibre reinforced composite comprising a matrix component and a fibre reinforcing component embodied within the matrix component; the fibre reinforcing component comprises arrays of fibres in which the fibres of each array extend in a predetermined direction in the matrix and in which the predetermined direction of the fibres of each array is different from that of the fibres of each of the other arrays; and the holes in the facing component part are so produced as to have a polygonal cross-section with sides arranged parallel to the predetermined directions of the fibres of the arrays.
Where the arrays of reinforcing fibres comprise first, second, third and fourth arrays of fibres having fibre directions of 0xc2x0, 90xc2x0 and +45xc2x0 and xe2x88x9245xc2x0, the holes in the facing component part have an octagonal cross-section with sides arranged parallel to the fibre directions.
Where the arrays of reinforcing fibres comprise first and second arrays of fibres having fibre directions of 0xc2x0 and 90xc2x0, or +45xc2x0 and xe2x88x9245xc2x0, the holes formed in the facing component part are so produced as to have a four sided polygonal cross-section with sides arranged parallel to the fibre directions.
According to a tenth aspect of the invention, there is provided a noise attenuation panel as hereinbefore set forth, wherein no holes are provided at locations of the facing component part which in the assembled panel are contiguous with the end portions of the walls of the cell defining wall structure.
According to an eleventh aspect of the present invention there is provided a noise attenuation panel as hereinbefore set forth, wherein the multiplicity of holes through the facing component part is in the form of an array of holes which pass through the component part from the rear face of the facing component part to the front face of the facing component part in a predetermined hole direction inclined to the normal to the front face.
The multiplicity of holes through the facing component part is preferably in the form of an array of holes which pass through the component part from the rear face of the facing component part to the front face of the facing component part in a predetermined hole direction inclined to the normal to the front face. The inclination of the holes is so chosen as to provide flow paths to the cell defining structure which optimise attenuating performance of the panel.
Where the facing component part is a multi-ply structure comprising a plurality of superposed ply elements, the holes may be so inclined as to offset structural weakness of the multi-ply structure in the region of the holes.
Where the panel is so located as to be subjected to gaseous fluid flow over the surface of the front face of the facing component part in a predetermined fluid flow direction, the predetermined hole direction has a component along the front face of the facing component part which is in the same direction as the predetermined fluid flow direction. Furthermore the holes may be so inclined as to reduce the tendency of the holes to become clogged by debris carried in the gaseous fluid flow over the front face of the facing component part.
Where the facing component part is in the form of a fibre reinforced composite comprising a matrix component and a fibre reinforcing component embodied within the matrix component, the laser drilling is carried out on the composite after precuring of the matrix component.
In an embodiment of the invention, the facing component part and the cellular component part are required to follow a predetermined contour for the panel when assembled, and the precuring of the facing component part is carried out to the predetermined contour for the panel.
Furthermore, panels produced in accordance with the invention may include facing component parts with holes having two or more of the hole characteristics called for in the aforementioned different aspects of the invention.